The Process Parameters of Injection Molding
Plastic injection molding is one of the most important polymer processing operations in the plastic industry today. The application of this process has increased significantly in many different industries and, in particular, automotive parts. Injection molding uses plastic in the form of pellets as a raw material. These pellets, or granules, are heated until melted and then injected into a mold where the material solidifies to obtain the desired shape. Finally, the mold is opened and, after cooling, the part is ejected.
Many studies have been conducted with the goal of improving and optimizing the process in order to produce the highest quality parts on a wide range of commercial plastic injection molding machines. Several process design parameters affect the quality of finished plastic parts. Our injection molding company adheres to exact parameters for our parts to be of impeccable quality. In this article, we look at the core parameters in the injection molding process and the importance of each.
Basic Process Parameters
Process parameters, which are also sometimes called process variables, are specific measures that refer to the status of the process. The values of the parameters indicate whether the process meets the plan or needs adjusting. Through empirical analysis, it has been determined that the most significant process parameters affecting the quality of the part being molded is the melt temperature, mold temperature, injection pressure, packing pressure, and holding time.
Temperature measurement and control are very important in plastic injection molding. Temperature affects every step of the injection molding process and must be properly managed. The plastic must gradually heat to ensure a smooth transition from solid to liquid. It is critical that the temperature is set up carefully so that the plastic is thoroughly melted and mixed, yet not allowed to burn.
The actual temperature needed is dependent on the material being used and the size and shape of the part being manufactured. A common range where the melt is formed is typically between 300 and 800 degrees Fahrenheit (149 and 426 degrees Celsius).
Most injection molding machines typically do not have enough temperature measuring points even though making measurements are relatively simple. Therefore, temperatures are sensed by thermocouples, which consist of two different wires joined at each end. A small electrical signal is generated if one end is made hotter than the other; the more the material is heated, the greater the signal. In addition, temperature controllers, installed on the injection molding machines, will regulate the temperature of the molding by the machine operator setting the required temperature. When the temperature reaches the set point, it turns off and the power is turned on again when the temperature drops.
The temperature of a mold is equally as important as the melt temperature; it affects quality in many ways. If a mold is used at an improper temperature, the part can result in defects like mold warpage, sink marks, and jetting.
Depending on the type of resin used, mold temperature can impact the properties of the part such as:
- Stress Cracks
- Creep Resistance
- Fatigue Resistance
- Wear Resistance
- Molecular Weight
- Dimensional Stability
Mold temperature refers to the temperature of the cavity surface when molding is performed. When the temperature distribution in the mold is uneven, the finished part can have uneven shrinkage and internal stress, which can make the molding mouth deformed and warped. The mold temperature is set from the lowest appropriate temperature of the material used and then adjusted appropriately according to the quality condition.
Injection pressure is a force applied to the molten plastic that results from the main hydraulic pressure pushing against the back end of the injection screw. This process pushes the molten plastic resin into the mold cavity. Pressure plays a significant role in the overall quality of a plastic part. Proper injection pressure keeps the mold closed during the injection process. Injection pressures must be high enough to fill the cavity, forcing material into the furthest reaches. The pressure balances the clamping pressure of the machine and is calculated according to the structure and dimension of the parts.
When there is too much or too little pressure, various issues result such as flashing and viscosity. Having an injection pressure that is too low inhibits the packing of the cavity because the material cools during the filling phase, causing the gate to freeze quickly after the mold is filled and leading to higher shrinkage. Excessive pressure can cause the mold to open and allow parting-line flash or damage an otherwise structurally sound mold.
Packing, or holding, pressure is the pressure that is maintained on the melt after the mold is filled and until the gate freezes or the pressure is removed by cycle time control. Packing pressure helps to ensure a dense part, molded with uniform pressure and controlled shrinkage. It can be defined as pressure against the cooling plastic in the cavity while that plastic solidifies.
Packing pressure is necessary for filling the remaining 5 percent of a mold tool cavity to prevent resin backflow and compensate for resin shrinkage caused during the cooling process. Appropriate holding pressure is critical for a high-quality part. If the holding pressure is set too high, the product will be prone to flash, overfilling, or stress near the gate. By the same token, if the holding pressure is too low, excessive shrinkage and dimensional instability will likely occur.
The holding stage is when the material is held in place at a pressure equilibrium until the gate freeze occurs, at which point the cooling process begins. Once the packing fills the entire mold, including the gate through which the material is being injected, the point of “gate freeze” is achieved. Gate freeze means that no more material can flow into the cavity or the gate and that the injection pressure is being held so that no material can backflow out of the cavity. This is a time-controlled phase.
Time for Injection, Holding, and Cooling
The injection time and cooling time are significant relative to the quality of injection molded parts. Mold filling time is generally not more than 10 seconds. The holding time typically takes a long time, with the amount of time being dependent on the wall thickness of the part being molded. Holding time should not be prolonged beyond the gate freezing time. The cooling time depends on the material thickness and mold temperature.
Contact Advanced Plastiform for an Injection Molding Quote
Our team at Advanced Plastiform, Inc. is hyper-vigilant in setting the correct parameters for producing the highest quality products. Our injection molding process is one of the best in the industry. Let us show you how we manufacture high-quality, durable plastics at a low per-unit cost and a fast lead time. We are dedicated to offering high-quality thermoforming and injection molding services. API is your answer for custom plastic products manufactured in North Carolina, South Carolina, Pennsylvania, Maryland, Tennessee, Georgia, and Virginia.